1. Field of the Invention
The present invention relates to a switching power supply, particularly for use as a DC--DC converter.
2. Description of the Related Art
FIG. 20 shows a main circuit of a switching power supply of flyback converter type. The circuit includes a transformer 6 having a primary coil 3, a secondary coil 4, and a third coil 5, a switching element 7 made up of a field effect transistor (FET), a current sensing circuit 9 made up of a resistor 8, an AC input power supply 10, a diode bridge circuit 11, a smoothing capacitor 12, a starting resistor 13, an IC-input capacitor 14, a diode 15, a switch control circuit 19 having an oscillator (OSC) 16, an RS flip-flop circuit 17, and a comparator 18 and integrated into a chip, a diode 20, a capacitor 21, and an output-voltage detecting circuit 27 having voltage-dividing resistors 22 and 23, an error amplifier 24, a photo-coupled isolator 25, and a reference power supply 26.
The operation of the circuit shown in FIG. 20 will be briefly described below by referring to a timing chart shown in FIGS. 21(a)-21(f). The OSC 16 in the switch control circuit 19 applies a constant-period pulse signal shown in FIG. 21 (d) to the set input terminal (S) of the RS flip-flop circuit 17. When the RS flip-flop circuit 17 receives the on-level output (set pulse) of the pulse signal from the OSC 16 at the set input terminal (S), it immediately outputs the on-level output of the pulse signal (gate-pulse signal) shown in FIG. 21 (f), from the output terminal (Q) to the gate G of the switching element 7. Then, the switching element 7 goes on due to the on-level output of the gate-pulse signal. When the switching element 7 is turned on, a current "i" based on the input power supply 10 and the charged voltage of the smoothing capacitor 12 flows through a path passing through the primary coil 3 of the transformer 6 and the current sensing circuit 9 (resistor 8). Electromagnetic energy is accumulated at the primary coil 3 due to the flow of the current "i." The current sensing circuit 9 converts the current "i" to a voltage, and outputs the voltage to the non-inverting input terminal of the comparator 18 as a detected voltage V.sub.CS shown in FIG. 21 (c).
During the switch-on period, the voltage across the capacitor 21 at the output side of the transformer 6 is output as the output voltage V.sub.out shown in FIG. 21 (a), and the output voltage V.sub.out is divided by the voltage-dividing resistors 22 and 23 in the output-voltage detecting circuit 27 and is input to the inverting input terminal of the error amplifier 24. A reference voltage output from the reference power supply 26 is applied to the non-inverting input terminal of the error amplifier 24. According to the difference between the divided voltage of the output voltage V.sub.out and the reference voltage of the reference power supply 26, the error amplifier 24 outputs the voltage V.sub.e shown in FIG. 21 (b) to the inverting input terminal of the comparator 18 through the photo-coupled isolator 25 in the output-voltage detecting circuit 27, as the detected voltage V.sub.f shown in FIG. 21 (c).
When the detected voltage V.sub.CS of the current sensing circuit 9 reaches the detected voltage V.sub.f of the output-voltage detecting circuit 27, the comparator 18 applies the on-level pulse signal (reset pulse) shown in FIG. 20(e) to the reset input terminal (R) of the RS flip-flop circuit 17. When the RS flip-flop circuit 17 receives the reset pulse, it immediately stops outputting the on-level gate-pulse signal to the switching element 7 as shown in FIG. 21 (f) to turn off the switching element 7.
When the switching element 7 is turned off, a current caused by energy accumulated in the transformer 6 is supplied as the output voltage V.sub.out via a loop passing through the secondary coil 4 and the diode 20. At the same time, a current caused by energy accumulated in the third coil 5 during the switch-on period flows through the diode 15 to the IC-input capacitor 14 to charge the capacitor. The circuit is then ready for the next switching on of the switching element 7.
When the output voltage V.sub.out rises from the specified voltage V.sub.a to a voltage V.sub.b as shown in FIG. 21 (a), for example, the output voltage V.sub.c of the error amplifier 24 in the output-voltage detecting circuit 27 becomes low as shown in FIG. 21 (b). Then, the detected voltage V.sub.f of the output-voltage detecting circuit 27 is lowered as shown in FIG. 21 (c), and the time period which is taken by the detected voltage V.sub.CS of the current sensing circuit 9 to reach the detected voltage V.sub.f becomes shorter. In other words, the time period (the switch-on period of the switching element), from when the RS flip-flop circuit 17 starts outputting an on-level signal to the switching element 7, to when the RS flip-flop circuit 17 receives a reset pulse from the comparator 18, becomes shorter, electromagnetic energy accumulated in the primary coil 3 is reduced, and the rise in the output voltage V.sub.out relative to the specified voltage V.sub.a is compensated for, a rise and V.sub.out is stabilized.
On the other hand, when the output voltage V.sub.out becomes lower than the specified voltage V.sub.a, contrary to the operation described above, the switch-on period of the switching element 7 is extended and electromagnetic energy accumulated in the primary coil 3 increases. The decrease in the output voltage V.sub.out relative to the specified voltage V.sub.2 is compensated for, and is stabilized.
The above-described control method is generally known as the current-mode control method, in which a current passing through the circuit is detected and converted to a voltage, the switch-on period of the switching element 7 is controlled according to the detected voltage and the output voltage, and the output voltage is stabilized. This control method is superior in response and provides stable control against changes in the output voltage V.sub.out compared with other methods such as the voltage-mode control method, since the circuit current is used in this method.
However, when the circuit has a light load or receives a high input voltage, a current "i" flowing through the circuit decreases and the ratio of a noise factor to the current "i" inevitably becomes very large. Namely, the S/N ratio of the current "i" largely declines. Since the current sensing circuit 9 converts the current "i," which has a decreased S/N ratio, into a voltage and outputs it as the detected voltage V.sub.CS, the S/N ratio of the detected voltage V.sub.CS is substantially reduced. The switch control circuit 19, which receives the detected voltage V.sub.CS, cannot correctly control switching on and off of the switching element 7 due to adverse effects of the noise factor in the detected voltage V.sub.CS. The output voltage V.sub.out is not positively stabilized.